Microfabrication techniques were originally developed for the microelectronic industry, researchers have been able to create simple designs such as well-defined and repetitive patterns of grooves, ridges, pits, and waves. Techniques such as photolithography, electron-beam lithography, colloidal lithography, electrospinning, and nanoimprinting are popular methods for fabricating micro and nano topographical features. The need for large capital investments and engineering expertise has prevented the widespread use of these fabrication methods in common biological laboratories.
Continual miniaturization of optical and optoelectronic devices drives the need for increasingly low cost, smaller form factor, and monolithic integration of versatile components such as microlens arrays (MLAs). Conventional micromachining to fabricate MLAs is limited in its scalability, with large area production becoming prohibitively expensive. As such, there is a corresponding interest in moving from glass to polymer MLAs. Many methods to fabricate MLAs in polymers have been demonstrated, and include such innovative techniques as photoresist reflow, laser ablation, and molding UV photocurable polymers from elastomer molds. Thermal photoresist reflow leverages surface tension to create hemispherical shaped lenses from melted micropatterned photoresist. While this is a pervasive method to create optical molds, this approach has a limited geometry that requires a certain thickness of photoresist. When the deposited photoresist thickness is too thin, significant deviations from a rounded shape ensues; therefore lenses with NA (numerical aperture)<0.15 are not possible. Laser ablation, for example with an excimer laser, can be used to create lenses in plastics such as polycarbonate. While this is an attractive direct write process, it is a slow serial process that requires precision instrumentation. On the other extreme, molding epoxies from elastomer molds such as polydimethylsiloxane (PDMS) allows for low cost replicate molding, but still needs creation of the original master.
Therefore, the ability to create large arrays of low cost microlens in a plastic substrate from scratch with acceptable optical properties remains a challenge. Photoresist, for example, has a relatively large absorption and is therefore not ideal for many applications. Transferring such features into optical grade plastic requires processes such as hot embossing, which necessitates both an electrode position process to create the metallic mold as well as expensive capital embossing equipment; this approach is therefore not amenable to prototyping and/or low volume production.